Hearing loss and vertigo are serious health problems that currently affect millions of Americans. Normal sensory function of the inner ear relies on mechanoreceptive hair cells, which convert sound vibrations and head movements into electrical signals that are conveyed to the brain. Hair cells can be injured or lost as a result of noise exposure, ototoxic medications, inner ear infections, or as part of normal aging. The inner ears of mammals (including humans) are unable to replace these cells, and the death of hair cells is the most common cause of sensorineural deafness. In contrast, the ears of nonmammalian vertebrates can quickly regenerate hair cells after injury, and it is of great interest to understand the cellular basis of this regenerative process. This proposal focuses on a novel and relatively unexplored aspect of regenerative biology. Prior research has shown that macrophages - the primary effector cells of the innate immune system - are critically involved in repair and regeneration of many types of somatic tissues. The cochlea and vestibular organs contain large numbers of macrophages, but the function of those cells within the ear is completely unknown. Our prior work has suggested that macrophages may help promote hair cell regeneration, but a direct demonstration has remained elusive. The proposed studies will address these long-standing issues, and will make use of newly-developed methods for selective elimination of hair cells and/or macrophages from the mature ear, both in vitro and in vivo. The outcome of the proposed experiments will indicate whether macrophages play a stimulatory or suppressive role in regeneration. Notably, knowledge acquired from this work will also extend beyond the realm of regenerative medicine. Many inner ear pathologies respond to immunosuppressive therapies. The cellular basis of such treatments is unknown, but these observations point to direct effects of inflammatory cells on the sensory structures of the ear. As such, completion of these projects will resolve many outstanding questions regarding the contribution of otic macrophages to sensory repair and will also provide new insights into the nature of the cellular dialogue between the innate immune system and the inner ear.